Goal: 

We should get dnsutils to no longer rely on Eventlet's concurrency model and/or monkey_patching, so that its execution environment is standardized on Python's native threading and is easier to debug and maintain.

Acceptance Criteria:

• Verify eventlet is removed from designate/dnsutils.py.
• Verify DNS zone transfer (XFR) operations and other utility functions correctly apply timeouts.
• Verify unit tests in designate/tests/unit/test_dnsutils.py (specifically{{ eventlet.Timeout }}usages) are replaced and still provide adequate coverage.

Examples for eventlet code usages in dnsutils files:

• designate/dnsutils.py
• designate/tests/unit/test_dnsutils.py